Infrared objectives are configured as pancratic systems to compensate for the defocusing effects of temperature fluctuations on the one hand, and to provide focal width changes for targets which move away rapidly such as flying objects.
It is known to displace the intermediate optical members of an infrared objective with the aid of a motor via gearing, threaded spindles and control cams. Such a pancratic infrared objective is disclosed, for example, in French patent publication 2,586,828. A disadvantage of this known IR-pancratic optical device is that the speed of displacement of the optical members is inadequate to sharply image a rapidly moving target such as a missile or a rocket as it moves along its path detectable through the entire focal length range.